Mixed ortho-, meta- and para- isomers of dichlorobenzene result from the nuclear chlorination of benzene or monochlorobenzene by either liquid or vapor phase processed. In liquid phase Friedel-Crafts catalyzed nuclear chlorination processes for the synthesis of dichlorobenzene from benzene or monochlorobenzene, the meta- isomer is typically formed at concentrations of less than 2.0 weight percent of the total dichlorobenzene content. The ortho- and para- isomers produced in the liquid phase process constitute the balance of the dichlorobenzenes and are generally present in a para- to ortho- weight ratio of between 3.5:1 to 1.4:1. Recycle of meta-rich dichlorobenzene isomer mixtures into the reaction zone with benzene and monochlorobenzene feedstock results in a gradual buildup of the meta- isomer in the reaction product.
Paradichlorobenzene and orthodichlorobenzene may be separated by known distillation and crystallization techniques, however the presence of meta- isomer increases the difficulty of such separations. For many purposes, dichlorobenzene products free of metadichlorobenzene are desirable. It is especially difficult to separate metadichlorobenzene from paradichlorobenzene because of the proximity of boiling points. In addition, enough of both ortho- and meta- isomer accompany the paradichlorobenzene fraction to allow their gradual buildup to significant levels when purification of paradichlorobenzene by crystallization is practiced. This buildup results in accumulation of a waste liquor of mixed dichlorobenzene isomers having low economic value.
U.S. Pat. No. 3,170,961 describes the removal of metachlorobenzene and orthodichlorobenzene from paradichlorobenzene by selective bromination. Orthodichlorobenzene is thereafter recovered by debromination.
To conveniently separate ortho- and para- isomers of dichlorobenzene by thermal fractionation and crystallization techniques, it is desirable that a process be found to significantly reduce the meta- isomer content of the dichlorobenzene mixture.